1. Field of the Invention
The present invention relates to an arch clamp foot lifting apparatus of a bar tacking machine.
2. Prior Art
FIG. 6 is a schematic view of a conventional bar tacking machine, which comprises an arch clamp device 22 which is disposed on an upper surface of a sewing machine bed 6, an arch clamp foot driving device 30 which is disposed in the sewing machine bed 6 for driving the arch clamp device 22 in the direction of X and Y axes, and an arch clamp foot lifting apparatus 33 which is disposed on the sewing machine arm 31 for driving the arch clamp foot 7 to move the arch clamp foot 7 up and down
In the conventional bar tacking machine, when a solenoid of a solenoid device 14 is excited to retract a plunger 14a into the solenoid device 14, a connecting link 50 is drawn rightward in FIG. 6 against resiliency of a return spring 51 so that upper and lower links 46 and 47 forming a shape of angle are extended up and down in the longitudinal direction thereof. As a result, a protrusion portion 40a of a retaining member 40, in its turn, a base end part of the retaining member 40 is pressed downward by a flange portion 5a of a presser member 5 to position lower than a pin 37 of an arch clamp foot lifting lever 38 so that the arch clamp foot lifting lever 38 is swung clockwise about the pin 37 against resiliency of an arch clamp foot spring 39, and hence the arch clamp foot 7 moves upward. When the arch clamp foot 7 moves upward, a material to be sewn between the arch clamp foot 7 and a throat plate 36 of a feed plate 23 is released from the clamping therebetween. There are provided a pin 48 which swingably connects the upper and lower links 46 and 47, a pin 70 which swingably supports the upper end of the upper link 46 on the sewing machine arm 31 and a pin 71 which swingably supports the lower end of the lower link 47 on the pressing member 5.
When the solenoid of the solenoid device 14 is released to the non-excitation position, the connecting link 50 and the plunger 14a are drawn leftward in FIG. 6 owing to resiliency of the return spring 51, and the upper and lower links 46 and 47 are bent in a shape of angle to reduce the length thereof in the up-and-down direction. Accordingly, the pressing member 5 is pulled upward and the flange portion 5a is brought into contact with a guide bush 43 attached to the sewing machine arm 31, thereby releasing the pressing state of the protrusion portion 40a by the flange portion 5a. As a result, the arch clamp foot lifting lever 38 which receives resiliency of the arch clamp foot spring 39 swings counterclockwise about the pin 37 to lower the arch clamp foot 7 so that the material to be sewn can be clamped between the arch clamp foot 7 and the throat plate 36.
When the arch clamp device 22 is moved on the upper surface of the sewing machine bed 6 in the direction of X and Y axes in a state where the material to be sewn is clamped between the arch clamp foot 7 and the throat plate 36 so that the material to be sewn is subjected to a given bar tacking stitches by a needle 49.
However, such a conventional arch clamp foot lifting apparatus has a construction that the pressing member 5 which is supported on the sewing machine arm 31 to move up and down is operated by the solenoid device 14 through the angle-shaped upper and lower links 46 and 47, so that the following problems occur.
FIG. 7 is a view for explaining the mutual motion between the angle-shaped upper and lower links 46 and 47, the plunger 14a of the solenoid device 14 and the pressing member 5. In the same figure, when a point a (a pin 48 to which a drawing force N is applied) of the angle-shaped upper and lower links 46 and 47 is drawn by the solenoid device 14 with the drawing force N, a pushing-down force W against resiliency P by the arch clamp foot spring 39 from the lower portion is generated on the pressing member 5 connected to a point b (pin 71). The drawing force N for generating the pushing-down force W for pushing down the point b against the resiliency P by the arch clamp foot spring 39 is sufficiently and abruptly small when the opening angle .theta. of the angle-shaped upper and lower links 46 and 47 approaches to zero. This is evident in view of the mechanism of the link. A linear line A shown in FIG. 7 shows a moving locus of the point a, and the point a which is drawn by the solenoid device 14 moves from A.sub.5 to A.sub.1. A.sub.0 is a crossing point between a line connecting the points b and c and the moving locus A. Accordingly, A.sub.5 corresponds to a pressing start position of the pressing member 5 and A.sub.1 corresponds to a pressing end position thereof.
As evident from FIG. 7, the pushing-down force W generated considering the increase of the resiliency P of the arch clamp foot spring 39 which is compressed when the pressing member 5 lowers and the drawing force N of the solenoid device 14 needed for generating the pushing-down force W have the following expressions.
N.sub.5 =W.sub.5 .times.tan.theta..sub.5 at the pressing start position A.sub.5.
N.sub.1 =W.sub.1 .times.tan.theta..sub.1 at the position close to the pressing end position A.sub.1.
Where W.sub.5 is a pressing force needed at the start of the pressing of the pressing member 5.
W.sub.1 is a pressing force needed at the end of the pressing of the pressing member 5.
Suppose that W.sub.5 :W.sub.1 =1:2 and .theta..sub.5 =25.degree. and .theta..sub.1 =5.degree., the following expressions are established.
N.sub.5 =0.466, and N.sub.1 =0.157
Suppose that the pushing-down force W has an equation expressed by W.sub.5 :W.sub.1 =1:2 considering the increase of the resiliency P of the arch clamp foot spring 39 which is compressed as the pressing member 5 lowers.
It is understood from the above that the maximum drawing force N.sub.5 (0.466) is needed at the pressing start position A.sub.5 of the pressing member 5, and the drawing force N.sub.1 (0.157), which is about one thirds of the maximum drawing force N.sub.5, is needed at the pressing end position A.sub.1.
In FIG. 8, a line T.sub.1 shows the drawing force N of the solenoid device 14 needed for pressing down the pressing member 5 from the pressing start position A5 to the pressing end position A.sub.1.
On the other hand, the actual drawing force N of the solenoid device 14 has the characteristics of a curve S.sub.1, and it increases when the plunger 14a retracts into the solenoid device 14. That is, the maximum drawing force N.sub.1 is generated at the final position (A.sub.1) of the retracting stroke of the plunger 14a. This runs counter to the drawing forces N.sub.5 and N.sub.1 of the solenoid device 14 needed based on the result of calculation wherein the excessive drawing force N.sub.1 is generated at the position close to the pressing end position A.sub.1 to satisfy the drawing force N.sub.5 at the position close to the pressing start position A.sub.5. As a result, energy is wasted.